Abstract-Engine knock is among the most relevant limiting factors in the improvement of the operation of spark ignited engines. Due to an abnormal combustion inside the cylinder chamber, it can cause performance worsening or even serious mechanical damage. Being the result of complex local chemical phenomena, knock turns out to have a significant random behaviour but the increasing availability of new on-board sensors permits a deeper understanding of its mechanism. The aim of this paper is to exploit in-cylinder pressure sensors to derive a knock estimator, based on the logistic regression technique. Thanks to the proposed approach it is possible to explicitly deal with knock random variability and to define the so-called margin (or distance) from the knocking condition, which has been recently proven to be an effective concept for innovative knock control strategies. In a model-based estimation fashion, two modelling approaches are compared: one relies on well-known physical mechanisms while the second exploits a principal component analysis to extract relevant pressure information, thus reducing the identification effort and improving the estimation performance.

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For internal combustion engines, it is important to ensure uniform cylinder-wise torque contributions in order to avoid excessive crankshaft torsional vibrations. Especially, the high-torsional vibration levels of mediumspeed power plants and marine engines cause unnecessary wear of mechanical components, such as the flexible coupling between the engine and the load. This is because of the fact that the lower torque-order frequencies excited by the fuel combustions are usually in the vicinity of the natural frequency of the flexible coupling. A cylinder-balancing method is presented, which minimises crankshaft torsional vibrations on medium-speed internal combustion engines. Using a model of engine dynamics, the reduction of crankshaft torsional vibrations can be addressed as an online optimisation problem, where the Newton direction of fuel-injection adjustments is determined at each step. The proposed method is tested on a six-cylinder 6 MW Wärtsilä power plant engine, showing that the torsional vibration level can be significantly reduced, well below admissible levels.

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